The invention relates to a method of and means for filtering near-stationary, relatively long duration noise from an input signal containing information such as speech or music.
For the purpose of the present application, the term "near-stationary, relatively long duration noise", is hereinafter referred to as noise of the type described, and refers to noise whose frequency spectrum does not vary with time or varies only within a narrow range over a predetermined period of time particularly with respect to the spectral parameters of the information. Vehicular and machinery noises are examples of noise of the type described. The minimum duration of noise of the type described is thus considerably longer than periods over which the spectrum of normal speech remains nearly fixed, such periods being of the order of a second. In addition, the invention is described below as being applied to filtering noise of the type described from speech, but the invention is also applicable to other information bearing signals.
U.S. Pat. No. 4,025,721 discloses a method of and means for adaptively filtering noise of the type described from speech by recognizing the existence of such noise, identifying its parameters, adjusting the parameters of a filter so as to filter such noise, and applying an input signal containing such noise and speech to the filter. Termination of the noise of the type described is also recognized in order to cause the input signal to then bypass the filter.
Basic to the disclosure of the above identified patent is the necessity for recognition, in an input signal, of pauses between speech intervals. When a pause containing noise of the type described is recognized, the parameters of the noise within the pause are identified and used to adjust the parameters of the filter. Subsequently, the input signal is applied to the filter which is effective to filter the noise from subsequent speech intervals.